To maximize the transmission capacity of an optical communication system, a single optical fiber may be used to carry multiple optical signals in what is called a wavelength division multiplexed system (hereinafter a WDM system). The multiple optical signals may be multiplexed to form an aggregate multiplexed signal or WDM signal with each of the multiple signals being modulated on separate wavelengths referred to as channels. Modern WDM systems have a high traffic capacity, for example, a capacity to carry 100 or more channels at 100 gigabits per second (hereinafter Gb/s) per channel, or more.
A WDM optical transmission system may include a transmitting terminal coupled to a receiving terminal through a relatively long link, e.g. a trunk or branch path, established by one or more segments of optical cable. Some systems, such as long-haul submarine systems, may have a length between terminals of thousands of kilometers and span large bodies of water (e.g. oceans). The terminals may be provided in cable stations that are separated by the body of water, and the cable is typically laid across the floor of the water body, e.g. the ocean floor. The optical cable includes one or more layers of insulation or armor, one or more power conductors, and a number of optical fibers for carrying WDM signals. At each end, the cable extends out of the water onto land, and eventually into a cable station.
At the cable station, the optical fibers in the cable are coupled to communication equipment capable of transmitting and/or receiving various signals between the terminals. Signals carried by the cable are received from originating destinations, and/or transmitted to final destinations, by land-based communication networks. The power conductor of the cable is coupled to power feed equipment at one or both cable stations for delivering power to repeaters coupled to the cable.
Increasingly subsea optical communication systems are limited by the ability to deliver power to subsea repeaters via a cable. When voltage is applied by power feed equipment at a cable station, the maximum voltage is at the cable station and voltage is gradually reduced as current flows along the cables and power is used in repeaters to amplify the optical signals. The maximum ability to power a link is thus limited by the maximum voltage that can be applied to the power conductor of the cable, i.e. the maximum cable voltage.